A power transmission apparatus for a vehicle may include: first, second, and third input shafts disposed coaxially with each other and each selectively connected with a motor/generator; a torque mediating shaft disposed coaxially with the first input shaft; a first intermediate shaft in parallel with the first input shaft; a second intermediate shaft in parallel with the first input shaft, and selectively connected with a transmission housing; a first shifting section including five gear sets, selectively receiving torques through the first and third input shafts, and providing intermediate shift-stages; and a second shifting section including a planetary gear set, forming an output torque by combination of torques from the first shifting section and the second input shaft, and outputting the output torque to the output shaft through one gear set.